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    Re: Reminder of total lunar eclipse tonight
    From: Frank Reed
    Date: 2014 Apr 14, 20:41 -0700

    We can navigate by lunar eclipse. Just as the first bite is taken out of the Moon (or the last fully sunlit illumination disappears, or the first illumination re-appears, or the last bite fades out), if you measure the Moon's altitude or the altitude of any star not near the meridian, you can then determine your longitude WITHOUT a chronometer. The eclipse events occur at the same instant of GMT (UT) for all observers. The star altitude yields LMT to compare against it. It's not "lunars" but it's based on the same principle of course. And how accurately can you time those eclipse events? Within a minute? Or two minutes?? Average the longitude determined from all four events, and you ought to get a longitude to within +/-15' or maybe a little better.

    Here on Conanicut Island, I can see the Moon and Mars, not far from it, but they are "fuzzy" in high cirrus haze, and we're expecting overcast for the actual event. No matter. Three more coming soon!

    By a razor-thin margin, Mars is at its closest approach to the Earth for a year and half tonight, almost simultaneous with the lunar eclipse. For another navigation-related trick, you can actually measure its HP (horizontal parallax) using a medium-power telescope. Measure the angular diameter of Mars tonight or any night, whether it's at its nearest or much further away. The angular diameter of Mars is almost exactly equal to its HP. For Venus, measure the angular diameter and divide by two. It's not too hard to see why this works when you remember that Venus is the same size as the Earth and Mars is about half the size of the Earth.


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